1. Field of the Invention
This invention relates generally to an ignition circuit for an internal combustion engine and, more particularly, to a multichannel ignition circuit for controlling the supply of current through a plurality of ignition coils when the internal combustion engine is operating at low RPM.
2. Discussion of the Related Art
In recent years, electronic ignition systems controlled by a microprocessor or control computer have been introduced into automotive vehicles. Because of this, complex mechanical parts have generally been replaced with solid state components that allow the microprocessor or control computer more precise control on the operation of the internal combustion engine.
In order to control the amount of current through the ignition coils in the ignition system, as well as the timing of the ignition firing sequence, the velocity of the flywheel in the internal combustion engine is generally monitored. At high RPM (revolutions per minute), generally greater than about 600 RPM, a predictable flywheel velocity enables proper timing for driving the ignition coils, as well as predictability on the amount of current flowing through the ignition coils. At lower RPM, generally less than about 600 RPM, unpredictable flywheel velocity occurs.
This unpredictability is a result of the flywheel not being heavy enough to operate at a constant angular velocity at lower RPM levels. Because of this, the control computer artificially causes the beginning of the timing pulses to occur earlier so as to ensure a high enough current level through the ignition coils for the firing sequence. However, since the velocity of the flywheel is unpredictable below 600 RPM, it is difficult to determine when top dead center (TDC) of a particular piston in a cylinder in the internal combustion engine is going to occur. This unpredictability may potentially cause the current in the ignition coils to be driven to high enough levels that could damage the ignition coils.
To prevent damage to the ignition coils, current limiting circuits are generally used to limit the current through the ignition coils. Generally, a current limiting circuit is used for each cylinder or ignition coil, thereby providing multiple current limiting circuits within the ignition system. This duplicated circuitry increases the overall cost, weight and complexity of the ignition circuit. The current limiting circuits in general may also cause early firing within a particular cylinder due to false voltage spikes or voltage peaks. Such voltage peaks are transferred through the primary windings of the ignition coil and subsequently amplified in the secondary windings to voltage levels which may cause early firing before top dead center (TDC) occurs.
What is needed then is a multichannel ignition circuit which does not suffer from the above-mentioned disadvantages. This will, in turn, provide a multichannel ignition circuit which reduces the overall weight, cost and complexity of the ignition circuit; replaces duplicated current limiting circuits with a single current limiting circuit that supports multiple channels; and provides peak voltage reduction circuitry to substantially eliminate any false voltage peaks within the ignition coils, thereby substantially reducing any misfiring. It is, therefore, an object of the present invention to provide a multichannel ignition circuit which improves upon the current deficiencies in existing ignition circuits.